In the Code Division Multiple Access (CDMA) system, a base station uses Pseudo Number (PN) sequences with different time offsets to modulate, wherein the PN sequences are generated by a 15 order shift register, and have 32768 phases, namely 32768 chips. Since the phases of two PN sequences which have difference of one chip are very close and the propagation space of a unit chip is smaller, a mobile station is very difficult to distinguish that signals come from which base station. In order to increase the isolation of different PN sequences, the interval between phases of available PN sequences is configured to be 64 chips. The available PN sequence is defined by its PN offset, thus there are at most 512 PN offsets in the network. Due to the complexity of propagation environment and other factors, the interval of 64 chips is still unable to make the mobile station distinguish base stations, so a parameter PILOT_INC is introduced for further increasing the isolation, that is, the number of the available PN offsets is 512/PILOT_INC, and thus the PN offsets are further reduced.
With increasing of the user amount, a large number of new base stations require to be built on the basis of the original CDMA network, which makes the phenomena of a confusion of the same PNs and/or a confusion of neighboring PNs to always occur, causes the network interference and mobile station dropping calls, and severely affects the network quality.
The confusion of neighboring PNs is that the mobile station is unable to distinguish at least two pilot signals of the PN offsets with the very close phase interval. For example, the mobile station establishes a call with a base station A (PN100); when the mobile station detects a pilot signal of a base station F (PN200) in a neighbor set, and since a propagation delay will cause offset of the phase of the PN short code, a pilot signal of a base station G (PN202) will fall into a search window of the base station F (PN200); and when the mobile station requests to establish a handover with the base station F (PN200), the mobile station can monitor the pilot signal of the base station G (PN202) from the search window of the base station F (PN200); and then if the Walsh code of a traffic channel established for the mobile station in the base station F (PN200) is used by a mobile station in the base station G (PN202), the mobile station in the base station F (PN200) will demodulate and combine the signals of the mobile station using this Walsh code in the base station G (PN202) according to the monitored pilot signal of the base station G (PN202), which results in higher frame error rate and even the problem of dropping calls.
How to identify the confusion of neighboring PNs existing in the existing CDMA network is exigently to be solved faced by the existing CDMA network.